docs: def ducrss(s1, s2):
      """
      Compute the unit vector parallel to the cross product of
      two 3-dimensional vectors and the derivative of this unit vector.
  
      http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/ducrss_c.html
  
      :param s1: Left hand state for cross product and derivative.
      :type s1: 6-Element Array of floats
      :param s2: Right hand state for cross product and derivative.
      :type s2: 6-Element Array of floats
      :return: Unit vector and derivative of the cross product.
      :rtype: 6-Element Array of floats
      """
      assert len(s1) is 6 and len(s2) is 6
      s1 = stypes.toDoubleVector(s1)
      s2 = stypes.toDoubleVector(s2)
      sout = stypes.emptyDoubleVector(6)
      libspice.ducrss_c(s1, s2, sout)
      return stypes.cVectorToPython(sout)